1. Field of the Invention
This invention pertains to a arrangements for transmitting data between two CPUs (central processing units).
2. Description of Related Art
Copying machines are one example of the possible environments where the present invention could be used. A conventional copying machine has a plurality of CPUs, two of which are the main control CPU and the scan CPU (for scanning an original with an optical system). The control and scan CPUs communicate through a "hand shake" system; that is, the main control CPU transmits a scan operation data block to the scan CPU, receives an answer from the scan CPU, and then transmits the next data block.
Within the copying machine context, typically the scan CPU must respond to the main control CPU within a time window of from 150 microseconds to 1.25 milliseconds. If a periphery device needs the attention of the scan CPU while the main control CPU is trying to communicate with the scan CPU, then the periphery device will send an interrupt signal to the scan CPU. The scan will then interrupt the main control CPU and the control CPU suspends communications. When the other functions have been attended to the scan CPU will clear the interrupt and send a return data block to the control CPU. Communication between the scan and control CPUs resumes.
But there are causes other than an interrupt from the scan CPU that may result in suspension of communications. There may be a failure in the communication line, preventing transmission of the return data block--for example, there may be a disconnection or a short circuit in the line. According to the current state of the art, the main control CPU will be unable to distinguish whether the communications interruption resulted from a routine interrupt from the scan CPU or from a more serious failure such as a defect in the communication line.
In view of this problem, an object of this invention is to provide a communication arrangement between CPUs in which, when communication is suspended, it will be possible to identify whether or not the cause of the suspension is due to a failure in the communication line.